I’ve exposed r.Lumen.Supported.SM5 in UE5-Main, which you can integrate in order to re-enable DX11 support. Maybe will be able to get that into a hotfix.
That said, DX12 is our main path for a long time now and has a few features like new GPU allocator, which allows to share memory between various temporary GPU allocations. It’s quite unlikely that DX11 would run faster. Most likely difference which you see is due to Nanite, VSM, Ray Tracing or some other feature being disabled when you switch to DX11. And if not, and you have a good proof, then it’s something we would want to fix on DX12 instead.